As a data output apparatus employed in, for instance, a word processor, personal computer, facsimile or the like, there is a printer which prints desired data, e.g., characters, images and so forth, on a sheet-type printing medium, e.g., paper, film or the like.
For a printing method for such a printer, various printing methods are known. Particularly, an inkjet printing method recently receives attention because of its capability to perform printing without contacting a printing medium such as paper, ease of color printing, and quiet printing operation. In general, such a printer widely adopts a serial printing method because of its low cost and ease of downsizing. According to the serial printing method, printing is performed by reciprocally scanning a carriage, including a printhead discharging ink in accordance with desired printing data, in a direction orthogonal to the printing medium conveyance direction.
Particularly in a thermal inkjet method employing a bubble generation of ink for discharging ink droplets, which is induced by thermal energy generated by sending an electric current to heaters contact with the ink for approximately several μ seconds, it is possible to form a large number of nozzles in the printhead at high density. Forming a large number of nozzles in the printhead is advantageous in terms of an improved printing speed.
However, in a printer employing a printhead which adopts the inkjet printing method, if a few number of nozzles among the large number of nozzles are found clogged or a wire to the heater is broken due to deterioration with age, ink droplet discharge cannot be performed. Such event interferes an image printing operation. In the event of finding such nozzle which is permanently unable to discharge ink droplets, conventionally the printhead is replaced to recover the normal printing operation of the printer. Depending on products, a user replaces such a damaged printhead. This facilitates the maintenance of the printer.
Furthermore, there is a printer, which uses a head cartridge integrally having a printhead and an ink tank, and allows replacement of the entire head cartridge when refilling ink. In such a head cartridge, although the running cost tends to be high, the printhead is kept in an excellent condition at all times. Considering that the running cost includes the cost of replacing a printhead, the replacement cost can be rewarded. In addition, the ability to perform high-quality printing with a suppressed occurrence frequency of a problem, e.g., a clogged nozzle due to deterioration with age and so forth, can be appreciated as an advantage.
FIG. 6 is a perspective view showing an example of a printing element unit integrated on a conventional printhead.
The printing element unit has a large number of nozzle orifices (discharge orifices) 402, including heaters, on a printing element base 401 formed with a semiconductor substrate. On the printing element base 401, although not shown in the drawing, heaters (electrothermal transducers) arranged at positions opposite to the discharge orifices and a driver circuit for sending an electric current to the heaters are arranged. Moreover, on the printing element base 401, a power supply terminal for supplying electric power to drive the driver circuit and a pad terminal 403 serving as a signal terminal are provided. In addition, on the printing element base 401, ink channels (not shown) for introducing ink to the nozzle orifices are provided.
The conventional printhead is structured to use one color of ink for one printing element unit, or use plural colors of ink to perform printing. Depending on a specification of a printer, the printhead may sometimes integrate a number of printing element units in accordance with the number of colors used in printing, e.g., three colors, four colors, or six colors. In a case where a printer, having a specification to perform printing with three colors of ink, employs a printing element which is structured to use one color of ink for one printing element unit, three printing element units are integrated to the printhead. In a case where a printer, having a specification to perform printing with six colors of ink, employs a printing element which is structured to use two colors of ink for one printing element unit, three printing element units are integrated to the printhead to enable six-color printing.
FIG. 7 is a schematic view of a printhead in which three of the printing element unit shown in FIG. 6 are arranged next to each other.
Referring to FIG. 7, reference numeral 501 denotes a supporting base formed with molded resin or the like for supporting the printing element, ink container and so on; 502, the printing element unit shown in FIG. 6; 503, an electric contact realized by wire bonding or the like for connecting the pad terminal of the printing element unit 502 to an external wiring; and 504, a flexible substrate.
The flexible substrate 504 is mounted on the supporting base 501, and electrically connected to a print substrate 506 through a folded portion 505. A plurality of head pads 507 are formed on the print substrate 506, and are electrically continuous with respective wirings of the flexible substrate 504 through wirings in the print substrate 506. The head pads 507 are provided to electrically connect with a printer main unit.
In the printing element unit 502, an electric circuit consisting of a transistor, diode, resistance and so on is formed inside a semiconductor substrate, serving as a base, by a semiconductor manufacturing process similar to a process of manufacturing an ordinary IC. Similar to the ordinary IC which has a low tolerance to static electricity, the printing element unit also has a low tolerance to static electricity.
An electrostatic tolerance, required by an ordinary IC, indicates a predetermined level of a static charge applied to a terminal, which does not cause a breakdown. The electrostatic tolerance is defined mostly with an electrostatic surge applied in a post-process of an IC production in mind, such as chip dicing from a wafer, package assembling, mounting an IC onto a substrate and so on. A generally required standard of the tolerance is, for instance, according to EIAJ standard, ±200 V at 200 pF and 0 Ω, or according to MIL standard, ±1.5 kV at 100 pF and 1.5 kΩ.
However, in the case of the printhead according to the present invention which is replaceable by a user, there might be a risk that a user who has not eliminated static electricity directly comes into contact with the electric contact (head pad) between the printhead and printer main unit. For this reason, the printhead requires a higher electrostatic tolerance than an ordinary IC.
The head pads of the printhead are electrically connected to the input pad of the printing element by a low-resistance wiring. In a case where the printing element incorporates a protection circuit similar to that of an ordinary IC, the printing element will have the same level of an electrostatic tolerance as that of the ordinary IC.
Inventors of this invention have experimentally manufactured a printhead with the use of a printing element incorporating a protection circuit similar to that of an ordinary IC, and performed an electrostatic test on the head pad of the printhead with an electrostatic surge caused by a human body in mind. As a result, the inventors have confirmed a breakdown of the printing element.
Particularly they have confirmed a high occurrence frequency of an electric insulation breakdown in an interlayer film, which consists of a silicon oxidized film and so on, disposed between the substrate and other wiring layers, in the neighborhood of the contact portion connecting a signal input pad of the printing element to a resistance portion that limits an electrostatic surge with a metal wiring. The cause thereof is in that, as the voltage suddenly increases in the pad terminal of the printing element due to the applied electrostatic surge, the potential of the diode provided subsequent to the pad terminal through a resistance portion has instantaneously exceeded the withstand voltage of the interlayer film before the surge is absorbed.
To improve the electrostatic tolerance, a countermeasure using a semiconductor manufacturing process may be considered. A withstand voltage of the interlayer film is substantially uniquely determined by the composition of the interlayer film, film nature, and film thickness. Therefore, countermeasures, such as changing the composition of the interlayer film, increasing the film thickness or the like, may improve the tolerance to a certain level.
However, in the thermal inkjet printhead which generates a bubble in ink by heating the heaters to discharge ink, increasing the film thickness of the interlayer film largely affects thermal conduction from the heaters to ink and ink discharge performance. For instance, the silicon oxidized film used as the interlayer film has a lower thermal conductivity than that of the silicon substrate constituting the base. If the film thickness of the interlayer film is increased, it becomes difficult for the heat generated by the heaters to transfer to the substrate, and ultimately a longer time is required for cooling the heaters.
The residual heat affects the subsequent bubble generation, and may cause printing quality deterioration such as a change of an ink-discharging amount. If printing is to be performed after sufficient cooling, a longer printing time is required due to the time necessary for cooling, and as a result, the printer performance deteriorates.
Furthermore, increasing the film thickness may cause disadvantages, such as a decline in a throughput of the film forming process in the semiconductor manufacturing process, or a negative influence on a device characteristic of the transistor or the like manufactured.
Furthermore, another countermeasure considered is to add a discrete device between the head pads of the printhead and printing element for anti-static electricity. However, because this countermeasure causes the increased number of components, it brings about disadvantages, such as an increased cost and size of the printhead.